The preparation of the glenoid during shoulder surgery to receive a plastic or metal implant relies upon the surgeon being able to identify the axis of the bone and locate the implant within the maximum bone stock. Access to the glenoid is limited by the depth of the structure in the wound and the surrounding tissue as well as the surgical technique used.
The anatomy of the glenoid and scapula is such that there is very little bone into which an implant can be placed. The articulating surface of the joint consists of a shallow dished cartilaginous area bonded by soft tissue of the rotator cuff which stabilizes the humeral head against the glenoid.
The glenoid is roughly oval to pear-shaped and most replacement implants follow this contouring for optimal fit. Whilst the lateral aspect of the glenoid presents a broad surface for articulation, the margins taper rapidly medially to form a narrow neck of about 10 to 15 mm thickness from which emerge the coracoid (superiorly) and acromial (posteriorly) processes. The thinning of the glenoid progresses into the scapula bone where the thickness may be as little as 2 mm to 3 mm, although there is a broader spine running along the inferior margin of the scapula.
The axis of the visible face of the glenoid is easily identified, but does not correspond to the axis of the glenoid neck which is oriented posteriorly behind the glenoid face. The amount of bone in the neck in line with the axis of the visible glenoid is therefore greatly reduced and so it is along the actual axis of the glenoid neck that the anchoring means of the implant needs to be positioned. In addition, disease or degeneration can severely restrict the size of the glenoid, making implant placement more problematic. Further the scapula is a floating bone which is highly mobile, presenting little resistance to pressure, particularly under anaesthesia.
There are several problems for the accurate and satisfactory placement of an implant including:
1. access to the glenoid; PA0 2. identification of the axis of the glenoid; PA0 3. the alignment of the implant against the glenoid face, PA0 4. the alignment of the anchoring means for the implant in the glenoid bone, and PA0 5. the risk of glenoid neck fracture or exposure and penetration of the anchoring means through the wall of the bone.
To date the placement of an implant has depended upon the surgeon visually assessing the axis of the glenoid. He can gain access to and see the glenoid. However, the deltoid muscle, tendons and other tissue obscure the glenoid neck and it is undesirable to disturb this region unnecessarily. Therefore, the surgeon has had little or no actual knowledge of the anatomy behind the face of the glenoid, particularly the thickness and orientation of the neck which as noted above does not correspond with the axis of the glenoid. There is therefore always a risk that the anchoring device for the implant will either impinge on the sloping back wall of the neck which will lead to unsatisfactory placement of the implant against the glenoid face, or will penetrate the cortical bone which may result in bone fracture or interference with or damage to soft tissue.
The invention therefore has been made with these factors in mind. The invention provides a glenoid alignment guide adapted to retract soft tissue and muscle away for the glenoid, and guide a drill bit into the glenoid in proper alignment with the glenoid neck and glenoid face of a patient.
Generally, the glenoid alignment guide comprises a retractor plate for displacing the posterior and superior aspect of the deltoid muscle, and a drill guide slidably carried on the retractor plate. The retractor plate has a curved finger narrower than the plate defining the forward end of the retractor plate. The finger is adapted for insertion behind the glenoid. The tip of the finger is arranged to engage the bone of the glenoid neck to form a fulcrum for levering the muscle and other tissue clear of the glenoid. The drill guide defines an axial path along which a drill bit is guided. The drill guide is movable along the retractor plate to a forward position for engagement with the glenoid face. Guide means (e.g., a guide hole) is provided in the drill guide for guiding a drill bit along the axial path to make a hole from the glenoid face into the glenoid neck along the axis of the glenoid neck to receive fastening or locating means of a glenoid component of a shoulder implant. The tip of the retractor plate when engaged against the bone of the glenoid neck aligns the axial path defined by the drill guide relative to the glenoid neck.
Preferably, the tip of the finger has serrations adapted for non-slip engagement with the neck of the glenoid, and the tip of the finger is spaced radially from the center of the axial path defined by the guide means by 1.5 mm to 2.5 mm plus the radius of the drill bit to be guided by the guide means. Most preferably, the tip of the finger is spaced radially from the center of the axial path of the guide means by 2.0 mm to 2.5 mm plus the radius of the drill bit to be guided by the guide means.
Also, preferably, the glenoid alignment guide is adapted among other things for improved viewing of the glenoid face. For example, the drill guide may have a periphery and a plurality of cut away portions around the periphery to improve the view of the glenoid face as the drill guide is advanced. This allows the glenoid face to be viewed through the cut away portions of the drill guide to facilitate centering the drill guide relative to the glenoid face.
Most preferably, the drill guide has a generally convex-curved frontal surface adapted to correspond to the generally concave-curved surface of the glenoid face of the glenoid.
In one preferred aspect, locking means is provided for releasably locking the drill guide against the glenoid face of the glenoid. For example, the locking means may comprise a rod projecting rearwardly from the drill guide, a pivotable locking lever having a hole with an elongate cross section through which the rod passes, and resilient urging means for urging the locking lever such that the rod is gripped by the edges of the hole. The arrangement is such that retraction of the drill guide is resisted by the gripping of the rod by the locking lever but is possible if the locking lever is manually pivoted against the urging force of the resilient urging means to release the rod.
Most preferably, the retractor plate has a track mounting the drill guide on the retractor plate for sliding movement of the drill guide along the track in the axial direction. The track constrains the drill guide from rotation relative to the retractor plate, with the drill guide being mounted on the rod to move axially together with the rod to allow advancing the drill guide along the track of the retractor plate by manually moving the rod. For example, the retractor plate has a main body having opposite side edges, and ribs extending from the main body along the opposite side edges to define the track.
In yet another aspect, the drill guide has a rear surface, and a collar is fitted around the drill bit for engaging the rear surface of the drill guide to limit the penetration of the drill bit into the glenoid bone.
Such a guide can assist the surgeon in both obtaining good exposure of the glenoid and location for drilling a hole for fixing an implant. Thus the finger can be relatively narrow and so damage to nerves and blood vessels behind the glenoid can be kept to a minimum as it is inserted and its tip, when engaged with the bone, will identify the thinnest part of the glenoid neck so aligning the drill guide for optimum drilling. Thus the neck of the glenoid is of reasonably consistent width in all patients and once the tip of the finger engages it, one can be sure that the drill guide will be well positioned and ensure that, when the surgeon drills a hole, it will pass along the bone in the neck and not penetrate through the neck.
In a further aspect of the invention, referring to the method of using the glenoid alignment guide, the retractor plate is inserted between the patient's soft tissue and muscle and the glenoid, and the tip of the finger is placed against the glenoid neck. With the tip of the finger against the glenoid neck, the tissue and muscle is levered aside to expose the glenoid face, by means of the retractor plate. Then, with the tip of the finger remaining against the glenoid neck, the drill guide is advanced to a position where the drill guide abuts the glenoid face. With the tip of the finger remaining against the glenoid neck and the drill guide abutting the glenoid face, the drill guide is used to align a drill bit for drilling a hole along the axial center of the glenoid neck to receive fixing means of the implant.